Spring Lock vs Tooth Lock Washers: Which Stainless Locking Washer to Use
Both split-ring spring lock washers and serrated tooth lock washers are sold as "locking washers," but they work through entirely different mechanisms, suit different joints, and carry different caveats. Choosing the wrong type does not always cause an immediate failure — it may simply give you false confidence in a joint that is not as secure as you think. This guide explains how each type works, where each is genuinely effective, and how the stainless steel grade decision fits in.
1. Split-Ring Spring Lock Washers (DIN 127)
The split-ring spring lock washer — standardised in DIN 127 (and ASME B18.21.1 for inch versions) — is a helical ring stamped from spring-grade strip and cut on a diagonal so the two ends are offset. When the bolt is tightened, the ring is compressed flat. The intended locking action comes from two sources: the stored spring energy pushes back against bolt relaxation, and the sharp, hardened edges of the split bite into the underside of the bolt head or nut and into the joint surface.
In practice, this mechanism has been studied extensively and its effectiveness in high-performance joints is debated. The Junker transverse vibration test — the standard method for evaluating vibration resistance of fasteners — consistently shows that split-ring washers offer limited resistance to loosening once the joint is properly preloaded. The prevailing view in fastener engineering is that the spring washer provides meaningful benefit primarily in joints where the bolt is not at full specified torque, where the spring energy compensates for slight under-tightening. In a correctly torqued joint, the two faces of the split are essentially flat, the spring action is minimal, and the washer behaves much like a plain washer with sharp edges.
That does not mean spring lock washers have no place. They are widely used in general-purpose machinery assembly, light structural work, and applications where assemblers are unlikely to maintain precise torque control, where moderate vibration is present and long-term safety criticality is low. They are inexpensive, available everywhere, and reasonably effective in that context. For high-vibration joints — engine mounts, structural connections subject to dynamic loading, safety-critical fasteners — consider using prevailing-torque (all-metal or nylon-insert) nuts, thread-locking adhesive, or fasteners with serrated bearing faces instead.
HUIHUI manufactures DIN 127 spring lock washers in 304, 316 and 316L. See the spring washers product page for details.
2. Tooth (Serrated) Lock Washers — Internal and External
Serrated lock washers — also called tooth lock washers or star washers — resist rotation through direct mechanical interference. The washer is stamped with a ring of radial teeth around the bore (internal) or the outside diameter (external). The teeth are formed at an angle; when the bolt is tightened, the teeth embed into both the bearing surface of the bolt head and the joint surface beneath the washer. Any tendency for the fastener to rotate in the loosening direction drives the angled teeth deeper, increasing resistance. Unlike the spring washer, this mechanism does not depend on stored energy — it depends on the teeth being harder than the mating surfaces and physically biting in.
Internal Tooth Washers
The teeth point inward toward the bolt bore. Because the teeth are fully under the bolt head or nut, internal tooth washers present a neat external appearance — no serrations are visible around the washer edge. They are the preferred choice where appearance matters (architectural or electrical panel work) or where clearance around the bolt is tight. The engagement radius is smaller, which means the effective torque resistance is slightly less than an external design of the same nominal size.
External Tooth Washers
The teeth point outward along the outside diameter. A larger pitch circle means more moment arm and, all else being equal, more resistance to rotation per unit of bolt preload. External tooth washers are the standard choice where maximum grip is the priority and cosmetics are secondary — machinery, brackets, junction boxes, and sheet-metal assemblies. Because the teeth are visible and accessible after assembly, their condition can also be checked.
Electrical Bonding and Grounding
Serrated lock washers are extensively used for electrical grounding and chassis bonding, and this is one application where they genuinely excel. The teeth cut through non-conductive surface treatments — paint, anodising, powder coat, zinc passivation, oxide films — and create direct metal-to-metal contact. A properly tightened tooth lock washer can establish a low-impedance bond path without requiring the assembler to scrape or abrade the surface first. External tooth washers are typically preferred for grounding because the larger tooth count and engagement area provide more contact points and a more consistent bond resistance. This application is common in solar mounting structures, switchgear enclosures, inverter chassis, automotive grounding straps, and industrial control panels.
3. Comparison: When to Use Which
| Type | How it locks | Best for | Notes / limitations |
|---|---|---|---|
| Split-ring spring lock (DIN 127) |
Spring tension under bolt head; sharp split edges bite into surfaces | General machinery assembly; moderate vibration; joints where full torque is not guaranteed | Locking benefit in correctly preloaded joints is limited per vibration testing. Not recommended as the primary locking method in high-vibration or safety-critical joints. |
| External tooth lock (serrated) |
Angled outer teeth embed into bolt head face and joint surface | Maximum grip on sheet metal; electrical grounding through coatings; general fastening where tooth marks are acceptable | Teeth damage soft surfaces (aluminium, brass, plastic). Teeth visible at joint edge. Requires harder surface than tooth material to be effective. |
| Internal tooth lock (serrated) |
Angled inner teeth embed under bolt head or nut bearing surface | Electrical panel and chassis bonding; neater appearance; tight external clearance | Slightly lower resistance torque than external type of same size (smaller engagement radius). Same surface hardness caveat applies. |
This table is a decision aid, not a design specification. For joints where loosening could cause injury or structural failure, consult a fastener engineer and specify an appropriate locking system with test evidence.
4. Stainless-Specific Considerations
Locking washers in stainless steel behave differently from their zinc-plated carbon-steel equivalents in a few important ways.
No plating to lose. Carbon-steel locking washers rely on a zinc or zinc-nickel coating for corrosion resistance. When the teeth embed and the coating is damaged, the bare steel beneath is exposed — this is a direct corrosion initiation site. Stainless locking washers have no plating; the corrosion resistance of the bulk alloy is present throughout the part, including at the tooth tips after they embed. This makes stainless the correct material for outdoor, wet, marine and food-grade joints regardless of locking washer type.
Grade selection: 304 vs 316. The same logic that governs flat washer selection applies directly here. 304 (A2) is correct for most indoor, architectural and moderate outdoor environments. 316 or 316L (A4) is the right specification when chloride is present — marine and coastal installations, de-icing salt exposure, chemical processing equipment, or any wet environment where pitting corrosion is a risk. 316L (low carbon) is the appropriate choice when the surrounding assembly involves welding; the reduced carbon content prevents sensitisation at the heat-affected zone. When matching to a bolt-and-nut assembly, specify the same grade throughout to avoid galvanic mismatch.
Work hardening and spring function. Stainless steel spring lock washers gain their spring characteristics through work hardening during the stamping and forming process — not through a separate heat treatment as used on carbon-steel spring wire. The spring rate is lower than a carbon-steel equivalent of the same geometry. This is worth noting when torque calculations assume a specific spring constant; if spring rate is a design input, confirm the stainless value with the manufacturer.
Galling. Austenitic stainless fasteners (304, 316) are prone to galling — cold welding under pressure and rotation — particularly when a stainless washer is torqued against a stainless nut or bolt. Using a lubricant (anti-seize paste or molybdenum disulphide) on the bearing surfaces reduces galling risk. This is relevant for both types of locking washer, though more critical for spring washers where the full tightening rotation occurs across the washer face.
5. HUIHUI Supply
HUIHUI has been manufacturing stainless steel washers at its Jiaxing factory since 2008. Both spring lock washers (DIN 127) and tooth lock washers — internal and external — are produced in 304, 316 and 316L on coil-fed presses. Standard DIN metric sizes are available from regular production stock; non-standard sizes and custom profiles can be produced to a customer drawing. ISO 9001:2015 (cert No. 74326Q00174R001) covers the full production process. Mill test certificates (MTC, EN 10204 3.1) tracing heat number, chemical composition and mechanical properties, and PMI by XRF, are available on request for export orders.
- Spring washers (DIN 127) — product page and specifications
- Tooth lock washers — internal and external — product page
FAQ
Do split-ring spring lock washers actually prevent loosening?
The locking value of split-ring (DIN 127) washers in properly torqued joints is a contested point in fastener engineering. The spring washer is intended to maintain clamp load by pushing back against bolt relaxation, and the sharp edges of the split are meant to bite into the mating surfaces. However, studies — including the widely cited Junker vibration test — have shown that once the correct bolt preload is achieved and maintained, the spring washer adds little additional resistance to rotation. In high-vibration or safety-critical joints, purpose-designed locking methods (thread-locking adhesive, prevailing-torque nuts, or fine-thread pitch) are generally more reliable. Spring washers remain in widespread use for general assembly where vibration is moderate and full preload is consistently applied.
What is the difference between internal and external tooth lock washers?
Internal tooth (serrated) lock washers have their teeth pointing inward toward the bolt bore. External tooth lock washers have their teeth pointing outward along the outside diameter. External teeth engage a larger diameter, so they provide more resistance torque and are generally preferred for maximum grip. Internal teeth are tucked under the bolt head or nut, making them less visible and preferred on finished or tight-clearance surfaces. Both types work by having the angled teeth bite into the joint surface under clamp load, resisting rotation through mechanical interference.
Can tooth lock washers be used for electrical grounding or bonding?
Yes — this is one of the most reliable uses for serrated lock washers in electrical and electronics work. The teeth cut through paint, anodising, oxide layers and powder coatings to make direct metal-to-metal contact, creating a low-resistance electrical path between the fastened component and the chassis or structure. External tooth washers are typically preferred for this application because the larger tooth engagement area gives a more consistent bonding contact. This use is common in panel-mount electronics, switchgear, solar inverter mounting, and automotive grounding.
Should I use 304 or 316 stainless for locking washers?
The same selection logic that applies to flat washers applies here. 304 (A2) is correct for general indoor, structural and most outdoor applications without chloride exposure. 316 or 316L (A4) is the right specification for marine, coastal, salt-spray, and chemical-process environments where chloride pitting is a risk. Because locking washers are hardened by forming (work-hardened during the stamping and tooth-forming process), they do not rely on surface plating for their spring or locking function — stainless is the right base material for corrosion-exposed joints. Match the washer grade to the grade of the bolt and nut to avoid galvanic mismatch.
Do you supply both spring and tooth lock washers in 304 and 316?
Yes. HUIHUI manufactures split-ring spring lock washers (DIN 127) and internal and external tooth lock washers in 304, 316 and 316L stainless steel. Standard DIN metric sizes are held in regular production; non-standard sizes and custom profiles can be produced to a drawing. Mill test certificates (MTC, EN 10204 3.1) and PMI are available on request. Send your specification to [email protected] or use the RFQ form and we will reply within one business day.
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